Nielsen Online recently released a study that found that time spent on social networks and online games is on the rise, while e-mail and instant messaging are on the down tick. According to the study, Internet users now spend 22.7 percent of their time on social networks—a 43 percent increase since June 2009—and are spending a mere 8.3 percent of their time on e-mail—a 28 percent drop since June 2009. Possible explanations?

The most immediate and most obvious is the meteoric rise of social networks. Facebook has easily gained critical mass, with everyone from your grandparents to newborn infants having Facebook pages. But not only that, social media has also been directly eating into the shares of email, music sites and instant messaging platforms. That’s because social media outlets now have these features built in. Facebook, for example, has its own chat feature and robust messaging system, which obviates the need for contacting someone via email, MSN Messenger, AOL Instant Messenger, etc. And because everyone’s on Facebook already, it makes sense to get in touch with them there.

The other operative trend is smart phones. Smart phones have now become the primary channel for quick, text-based contacts. Whether it’s from a mobile email program or via a SMS message, more people are turning to smart phones for back-and-forth contacts. This can be done from anywhere—on line at the bank, standing on a bus or riding an elevator—which means users may spend less time sitting at their actual PC logging into the web-based interfaces for email.

One last interesting finding from this study: portals have taken a significant hit as well. Portals, such as iGoogle, MSN and customized Yahoo pages, have long served as the home base for many an Internet user, since they pulled together all the useful web from the data, including news stories, email messages, recommended websites, search engines and more. But again, social media appears to be filling that niche. Facebook, MySpace and Twitter themselves can be viewed as portals—after all, they are prolific sources of article recommendations, friend updates and conversation starters.

The lesson for Internet marketers? Focus on social media. Whether that means buying ad space that run down the right-hand column on Facebook or actively engaging contacts with your own fan page or Twitter account is up to you. But social networks are now the official hang out of today’s Internet users.